A tread pattern employed for many conventional winter tires, such as studless tires, is a block pattern with plural sipes formed in each block so as to extend in the tire-width direction (see, for example, Patent Document 1).
Decreasing the ratio of groove area thereby increasing the contact area is an effective method of improving on-ice performance of the tire of the above-mentioned type. There is, however, a problem that simply reducing the ratio of groove area may impair the drain function and thereby lower the wet performance. To put it differently, there is usually a trade-off relationship between good on-ice performance and good wet performance.    Patent Document 1: Japanese patent application Kokai publication No. 2005-349970